As background, many techniques have been developed to encode or modulate information onto a waveform for the purpose of transmitting said information in a communication system. Some of these techniques have been developed in order to improve the transmission of the waveform, such as increasing the power efficiency of the transmission amplifier (which may improve battery life in battery-powered systems) or reducing the cost of the amplifier. Other techniques have been developed in order to improve reception of the waveform, such as reducing the sensitivity of the receiver due to timing error and/or multipath which may be experienced when the waveform is propagated over-the-air. The CG-CPFSK systems and methods described herein are capable of improving both the transmission and reception of the waveform.
A known technique for encoding information onto a waveform is called continuous-phase modulation (CPM), in which a single carrier waveform is created that has a substantially constant modulus or amplitude. CPM generally includes direct phase modulation and continuous-phase frequency-shift keying (CPFSK) modulation. An advantage of this technique is that is works through saturating a power efficient transmission amplifier and is, therefore, capable of achieving relatively efficient power amplification during transmission of the waveform. A primary drawback is that the signal is vulnerable to distortion caused by the transmission channel (e.g., multipath), either suffers in detection efficiency, or requires a complex detector to mitigate the imposed channel distortion.
Another known technique is called orthogonal frequency division multiplexing (OFDM), which establishes modulation symbols in the frequency domain and transforms them (via discrete Fourier transform, or DFT) to a form in the time domain. A single frame contains several frequency-multiplexed symbols simultaneously that are the sum of sinusoids. The frames are cyclic in nature, meaning that the resulting signal out of the inverse DFT represents a waveform that is continuous from one end of the frame to the other. This results in a bandwidth-efficient waveform structure that may be extended in time (called a guard interval) in order to guard against transmission channel transients and to also mitigate the complexity of equalization. A primary disadvantage of OFDM is that the signal has a high level of amplitude variability and requires a linear amplifier with substantial backoff and is, therefore, inefficient.
A known variation of OFDM, called constant envelope OFDM (CE-OFDM), sends the OFDM signal into a phase modulator; the phase of the signal is defined by a real baseband phase function from a constrained OFDM modulator. The resulting waveform is a constant-envelope signal. A primary drawback of this approach is that the resulting signal has poor detection efficiency.
Yet another known technique is called single-carrier frequency division multiple access (SC-FDMA), which uses a small DFT and larger inverse discrete Fourier transform (IDFT) in combination to create a cyclic time-domain waveform that is the superposition of pulses defined inherently by the DFT and subsequent frequency-domain weighting process. Since the result is cyclic, a guard interval may be formed, as with the OFDM technique. Multiple users may be multiplexed onto SC-FDMA either at baseband or via careful alignment in the radio frequency (RF) channel. The amplitude variability of SC-FDMA is generally less than that of OFDM because the SC-FDMA technique basically creates a cyclic, partial response filtered data pulses in the time domain.
Still another known technique for encoding information onto a waveform is called Nyquist cyclic modulation (NCM), which is a variant of SC-FDMA. The NCM technique uses all the channels for one user and supplies a diverse set of partial response filter variants.
Yet another known technique is called continuous-phase modulation, single-carrier frequency division multiple access (CPM-SC-FDMA), which was developed to reduce the dynamic range of the waveform. In this technique, samples of a CPM waveform are fed into a configuration of the SC-FDMA modulator, and the resulting waveform has reduced amplitude variability. However, the resultant waveform is not CPM (i.e., not constant modulus); only the input is CPM.
Lastly, there is a technique called constant envelope, single-carrier frequency division multiple access (CE-SC-FDMA) that applies a SC-FDMA modulator and sends the output into a phase modulator similar to CE-OFDM. As with CE-OFDM, CE-SC-FDMA has poor detection efficiency.
The CG-CPFSK systems and methods disclosed herein overcome the inherent disadvantages in each of the known techniques by scaling the output of a SC-FMDA modulator to establish the appropriate phase area under a cyclic frame of SC-FDMA pulses. This output is then fed into a numerically controlled oscillator (NCO) or similar apparatus which converts the output value into a frequency-modulated waveform. Only the underlying phase function is cyclic; the resulting waveform is not cyclic but has a phase trellis that is reliably terminated. Also, the modulated waveform may have a substantially constant modulus and no discontinuities in one or more embodiments. Such a modulated waveform permits the use of power-efficient, low-cost amplifiers for the transmitter, while reducing signal sensitivity due to timing error and multipath for the receiver. Note that, in the technique disclosed herein, the SC-FDMA modulator is configured to establish a signal with particular properties that affect frequency changes out of the NCO; this substantially differentiates this system from CE-SC-FDMA.